Characterizing Chaos in a Giant Oil Field – Reservoir Architecture of the Horseshoe Atoll Trend, Late Carboniferous (peak icehouse) of the Permian Basin

Charles Kerans
Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin

The Pennsylvanian-Permian Horseshoe Atoll is one of the most prolific trends in the Permian Basin of West Texas with an OOIP of greater than 4 Bbbl. It is now one of the most dynamic of the developing reservoir trends in the world, with tertiary recovery programs either currently operating or being initiated in the Sacroc, Cogdell, Salt Creek, and Sharon Ridge units. The success or failure of these enhanced recovery programs depends on an understanding of the interaction of deposition, erosion, and karstification during periods of peak eustatic amplitude fluctuations.

The Northern Platform of the Sacroc Unit and the entire Cogdell Unit are contiguous producing portions of the Horseshoe Atoll including more than 1000 wells. Additional data used for the study include 36 sq mi of 3D seismic data, approximately 25 cores, and limited production data. 

The study illustrates the combined influence of regional tectonism, evolving icehouse eustacy, and associated physical and chemical erosion as well as depositional asymmetry associated with windward-leeward depositional patterns. Each reservoir unit can be broken down into cyclic platform-top strata, slope breccias and turbidites, stacked oolitic shoals and associated slope fans, and paleokarst systems. Reservoir characterization in such a complex system is best handled by treating each specific area of a unit as a special case and developing custom reservoir models accordingly.